In recent years, imaging devices, such as various cameras, have been sold from manufacturers. However, such imaging devices from the manufacturers are not standardized in hardware specifications and development process, which may result in color information of captured image data not being able to be handled as the same color value.
For example, an imaging device converts color information of captured image data, which is a captured color image of a subject, into an RGB (Red, Green, Blue) signal represented by the three primary colors of light, and then outputs the RGB signal to an image output device, such as a display device and a printing device (printer). Such an image output device generally reproduces a color image based on a signal value of the supplied (inputted) RGB signal.
The RGB signal obtained by the imaging device, however, depends on hardware properties, including spectral transmittance of an optical system such as an imaging lens, and of an RGB filter (filter of the RGB primary color system), and software image processing, such as white balance adjustment, gamma correction, and tone mapping.
Further, such image output devices also have various reproduction methods and individual differences. Due to these differences, even when the same color information is inputted at a spectral level for output of an image captured by such an imaging device from an image output device, the color actually reproduced (signal value of RGB signal) may be different from that of the captured image, resulting in failure of correct color reproduction.
To cope with such a situation, a standardized color signal in accordance with sRGB (standard RGB) and the like is often employed for passing captured image data between the imaging device and the image output device. In this case, in the imaging device, an RGB signal recorded in a CCD (charge coupled device) and the like by an imaging system is subjected to color correction in accordance with the sRGB standard and to output captured image data including the color corrected color information of the sRGB standard. With this configuration, an sRGB standard compliant image output device achieves correct color reproduction of a color image using sRGB standard compliant captured image data supplied from (outputted from) an imaging device.
There is a plurality of such techniques of color correction in accordance with the sRGB standard. For example, a configuration is proposed that approximates reproduced color in color reproduction by optically configuring spectral properties of an optical filter provided in the imaging device to match the sRGB standard.
Each manufacturer, however, often produces an original optical filter with various properties for improvement in image quality. Therefore, it is realistically difficult to get spectral properties of such an optical filter to fully optically match with the sRGB standard.
Similarly, in image processing to generate captured image data of the sRGB standard from captured RAW data, each manufacturer often applies processing to produce an image which is natural in appearance using an original image processing engine, and thus correct color reproduction of a color image is also difficult.
Meanwhile, a generally used method approximates reproduced color to target color by electrical correction, in which an RGB signal is subjected to matrix calculation (see, for example, PTLs 1 and 2).
The above electrical color correction method where an RGB signal is subjected to matrix calculation creates a profile of hardware properties for each imaging device using RAW data capturing a color chart, such as a Macbeth color chart, for color calibration to correct differences between various imaging devices.
As described above, estimation of color properties of an imaging device using a color chart allows correct color reproduction by the imaging device.